Monday, August 8, 2011

The Flandria Fixed Gear (Fixie) Conversion Project - Gearing

The proper gearing is the biggest decision that you are going to have to make when making a fixed gear bike. And the best way to make this decision is on a multi-speed bike. Go ride where you plan to ride your fixed gear and try to ride around in different gear combinations till you find the one you like the best. Once you agree on the gear ratio you liked the best write it down (# of teeth on front crank - # of teeth on rear sprocket).

The other way to figure out what gears you want in the front and back is to use a specific Gear Ratio or Gain Ratio (good article by The Sheldon Brown).

Gear Ratio (old way, common, not universal but close enough) is pretty simple : # of teeth on front crank / # of teeth on rear sprocket. This measurement requires that when comparing bikes with the same Gear Ratio they also have to have the same wheel and crank size. And this is not always true. So then came along the Gain Ratio (more accurate, but not really needed since the change is usually only a few percent difference between bikes)

EX: 52 teeth on chain ring, 19 teeth on rear sprocket = 52/19 = 2.75 Gear Ratio.

Now a lot of people will ask what ratio they should use. This is not easily answered because it is based on fitness level, riding terrain and style. (That is why I suggest my multi-speed test above). But here is a general guideline for Gear Ratios (The lower the Gear Ratio the easier to pedal):

  • Hilly or Cruising (when you want more style than speed) - 2.5- Speed at 80 RPM = 16 MPH
  • Average (some hills, some flat, cruising, BMX) - 2.75 - Speed at 80 RPM = 17 MPH
  • Super Flat (or City Riding with big acceleration) - 3.00 - Speed at 80 RPM = 19 MPH
And if this is all too much math for you use the web program I use to calculate Gear Ratio - Rabbit

Once you choose your Gear Ratio then you have to decide what chain ring to use. Since 36/12 and 45/15 both give you a Gear Ratio of 3. But SIZE MATTERS. Big Chain Ring - Less wear on chain and sprocket, but a bit heavier than a Smaller Chain Ring.

I went with a 52 - 19 (Gear Ratio of 2.75) as this allows me to ride around downtown and still get up the hills to North Park.

Next Post will be about how to install the proper cog onto your rear wheel or hub.

Step 1 - Background, Disassemble
Step 2 - Frame Painting
Step 3 - Handlebar & Fork Assembly
Step 4 - Gearing (Gear Ratio)
Step 5 - Wheels and Hubs
Step 6 - Custom Leather Saddle
Step 7 - Custom Leather Handlebars

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